Method of bleaching moist felted groundwood pulp



' of the ligneous material.

Patented July 11, 1950 METHOD OF BLEACHING MOIST FEL'IED GROUNDWOOD PULP Robert L. McEwen, Williamsville, and Fred R.

Sheldon, Buffalo, N. Y., assignors to Buflalo- Electro Chemical Company, Inc., Buffalo, N. Y., a corporation of New York No Drawing. Application July 30, 1947,

l 2 Claims.

This invention relates to a paper furnish constituent and method of making the furnish and more particularly to brightening or bleaching wood pulp containing substantial amounts of ligneous and non-cellulosic materials, such as groundwood or mechanical pulps, pretreated groundwoods or semi-chemical wood pulps or mixtures containing such types of pulps.

Brightening and obtaining color stability of wood pulps containing substantial amounts of ligneous and non-cellulosic material present special difliculties due primarily to the presence It is well known that chemical wood pulps contain very small amounts of lignin generally below about 3-4% of the total weight. On the other hand, groundwood, or mechanical pulps, pretreated groundwood and semichemical wood pulps contains up to about 35% lignin based on the weight of the pulp. The present invention relates to wood pulps containing lignin in amounts greater than about 5-6% by weight and are referred to herein as pulps containing substantial amounts of ligneous and noncellulosic material. groundwood or mechanical pulps, pretreated groundwood or semi-chemical wood pulps or mixtures of these wood pulps with other pulps.

As is well known in the art, wood pulps containing substantial amounts of ligneous and noncellulosic materials either in the bleached or unbleached stage, have a definite tendency to color reversion or darkening on storage, which limits the commercial use and desirability of such pulps. This tendency is particularly pronounced when the normal peroxide bleaching chemicals are left in the pulp. Various methods and techniques have been proposed for reducing the color reversion or impermanence of these bleached pulps but each of these proposed methods has involved elaborate, carefully controlled and costly processes, chemical additions, and equipment which when employed on a commercial scale greatly increase the cost of producing the pulp. Thus, where such pulps containing substantial amounts of ligneous and non-cellulosic material have been bleached by the addition of an alkaline peroxide solution, not only was the bleach solution applied and the impregnated pulp maintained under carefully controlled conditions, but also costly subsequent processing to prevent color reversion was necessary. This is particularly true in so called high density pulp bleaching where consistencies are over about 25%. Such subsequent treatment was required to terminate the bleaching reaction at the point of maximum brightness Such wood pulps can be- Serial No. 764,902

since, if the chemical reaction were allowed to continue, rapid color reversion would result. Such subsequent treatment involved washing, acidifying or neutralizing the bleached pulp, all of which resulted in additional costly processing in regards to materials, equipment, supervision, labor and time required. Further, such washing, acidifying or neutralizing very often again necessitated thickening the pulp to provide commercial lap, web or crumbs, for storage, transport, or sa e.

One of the principal objects of the present invention is to provide a method of making a paper furnish from bleached wool pulps containing substantial amountsof ligneous and non-cellulosic materials which can be carried out at a low cost commensurate with the relatively small increase in price which bleached groundwood or semichemical pulps command.

Another principal object of the present invention is to provide a simple method in which the paper furnish constituent and paper composed of such wood pulp containing substantial amounts of ligneous and non-cellulosic material has a brightness equally as high as that obtainable with more costly methods of brightening such pulp.

Another object of the invention is to provide such a process which eliminates the need for any washing or chemical processing subsequent to the application of the bleaching solution to the pulp and prior to the addition of the pulp to the furnish beater.

Another object is to provide such a method which possesses a high degree of brightening efficie'ncy with a given amount of bleach solution.

Another object is to provide a furnish constituent for use in paper making in the form of a moist felted wood pulp containing substantial amounts of ligneous and non-cellulosic material, capable of being handled without physical disintegration, and which is brightened by a slow internal bleaching action, permitting such bleaching action to take place while the furnish constituent is in storage at the mill or in trasit.

Another object is to provide a furnish constituent comprising wood pulp containing substantial amounts of ligneous and non-cellulosic materials which can be accumulated and stored in a moist condition for reasonably variable and flexible periods of time in accordance with present commercial practice, without substantial brigthness reversion,

Another object is to provide such a furnish constituent for use in paper making which reaches maximum brightness within a period of from about iiv hours to several days and which maintains this brightness without substantial brightness reversion or darkening for a period of several weeks thereafter, thereby permitting the furnish constituent to be stored or remain in transit for reasonable periods.

A further object is to conserve the nation's raw material resources by providing an-economical, simple and practical method of bleaching or brightening the'high yield but impure groundwood or semi-chemical pulps.

Another object is to provide such a process which does not require any substantial addition to or alteration of the equipment now used in producing and handling lap, web or crumbs, thereby not only reducing the capital investment required in brightening groundwood, pretreated groundwood or semi-chemical pulps, but also reducing the costly maintenance of pulp bleaching equipment.

Another object is to provide such a process which does not require extra steps in the handling of the material and is carried out as a continuous bleach process synchronized with the making of the high density lap, web or crumbs.

Another object is to provide such a process which is adapted to small and remotely located pulp mills, the bleaching solution being readily applied at the pulp mill and the bleaching reaction taking place while the lap, web or pulp is' being transported to the paper mill.

Other objects and advantages will be apparent from the following description:

Wood pulps containing substantial amounts of ligneous and non-cellulosic materials, such as groundwood, pre-treated groundwood and semichemical pulps, produced by conventional methods are employed in the practice of the present invention. These pulps when screened and generally at consistency of from about 0.5-'-6.0% solids, the balance being water, are dewatered on conventional mill apparatus to provide a moist felted pulp capable of being handled without physical disintegration. The consistency of such moist felted pulp is increased to n t less than 25% solids and preferably 35% to 50% by the dewatering treatment. While the moist-felted high density pulp can be in the form of lap, web or crumbs, in the following description such pulp shall be refered to as lap.

In the practice of the present invention, the

bleaching or brightening of the pulp is effected by an alkaline peroxide solution. The moist felted pulp is impregnated with this alkaline peroxide solution in any suitable manner, such as by padding or spraying. The bleach solution is preferably padded on the moist high density felted pulp or lap by applicator rolls; as it leaves the vacuum thickener, feltless "wet machine, or

other dewatering apparatus.

- The bleach solution is added to the moist felted high density pulp preferably in an amount insufllcient to materially reduce the consistency'of the pulp but sufficient to permeate the pu p. It has been found that an amount of the bleach solution added which lowers the consistency of the pulp as little as 2 or 3%, is sufficient to bleach throughout the pulp lap satisfactorily.

Following the application of the bleach solution to the moist felted high density pulp, the impregnated pulp is accumulated into a compact mass for storage or shipment. In this massed condition the impregnated pulp has a substantial volume and relatively small exposed area so that no substantial evaporation or temperature change is likely to take place. In this massed condition the impregnated pulp can be stored or be in shipment for any reasonable period of time. During this time of storage or shipment the bleaching 5 reaction takes place and also the presence of the residual bleaching chemicals is not detrimental to the brightness stability. At any time after maximum brightness has been reached, the impregnated pulp can be added directly, without further treatment or processing, to the other paper furnish constituents. It will therefore be seen that in eflect the pulp is produced as lap, is impregnated with the bleaching solution as lab. is allowed to bleach as lap, maintains its brightness stability as lap, is stored or shipped as lap. and is supplied as lap for furnish preparation. This is in distinct contrast to prior practices of high density bleaching where, after the bleaching and before rethickening, redilution and washing, acidifying, or neutralizing was required.

The alkaline peroxide solution employed in the practice of the present invention comprises an aqueous solution of a peroxide and an alkaline salt. The alkaline salt renders the high density pulp to which the solution is applied initially alkaline and also controls the lowering of the pH and total alkalinity of the impregnated pulp. Any peroxide which produces hydrogen peroxide in aqueous solution may be used. These may include sodium peroxide, sodium perborate, barium peroxide; et al. The alkaline salt can be any one selected from the group consisting of silicate of soda, trisodium phosphate, sodium pyrophosphate, sodium tetraphosphate, or other high pH salt having similar properties, either alone or in combination with one anotheror in combination with other alkaline materials such as caustic soda, soda ash, borax, and the like. The initial pH of the alkaline bleach solution before application to groundwood or pretreated groundwood pulp can be in the range of about 9.5 to 11 with pH 10.5 to 10.8 being preferred.v However, with semi-chemical pulps, a higher pH range has been found to be desirable, that is, the initial solution pH may be as high as 11.5 with a preferred pH range between about 10.8 and 11.2. The initial total alkalinity of the solution, regardless of whether the pulp treated is groundwood, pretreated groundwood or semichemical pulp, can vary from 0.35 to 1.1% by weight sodium oxide equivalent based on bone dry pulp. The bleach solution is applied to the pulp at the usual temperature existing in the mill. While this temperature varies from mill to mill and for different 68 seasonal conditions in a particular mill, the temperature generally varies in the range of 15 to After the pulp has been impregnated with the bleach solution, the impregnated pulp is accumulated into a mass of substantial volume and having a relatively small exposed area to inhibit drying' thereof. This can be effected by merely lapping, stacking, piling, or baling, or in any other suitable manner. when accumulated in this manner, the impregnated moist ulp has little exposed area and is not subject to substantial evaporation or temperature change'so that a slow internal stabilized brightening reaction takes place under substantially constant condiitions of pulp density and temperature.

In this accumulated condition of the impregnated pulp a slow internal bleaching actiontakes place until maximumbrightness is reached; The period between the time of application of the bleach solution to the pulp and the point at which maximum brightness will be substantially maintained without substantial brightness reversion or darkening for a period of several weeks thereafter. This period of time during which the maximum brightness is substantially maintained is generally ample to cover the shipping and storage time now normally encountered between the formation of the lap and the use of the lap in paper making so that the practice of the invention requires little or no alteration in present commercial practices in making, storing, shipping and using the lap, the stabilized bleaching reaction taking place during the usual conditions now encountered in handling lap for paper making.

To eifect a bleaching of the moist impregnated pulp to maximum brightness and to thereafter prevent reversion in brightness for the period of time indicated, it has been found that the initial pH of the bleaching solution and the total amount of alkali therein is critical. In connection with the total alkalinity of the alkaline bleaching solution used in the practice of the present invention, it has been found that if the total alkalinity of the solution'is below 0.35% by weight sodium oxide equivalent based on bone dry pulp, sufficient increase in brightness is not obtained, and, on the other hand, if the bleaching solution contains more than 1.1% total alkalinity on the same basis, while sufficient increase in brightness is initially obtained, this is followed by a rapid darkening of thepulp which renders the pulp incapable of being allowed to stand for the periods of time permitted by the present invention and requires the use of a neutralizing agent at the point of maximum brightness in order to retain this brightness. It has been found that, with ground-wood and pretreated groundwood, the peroxide bleaching solution containing the amount and type of alkali above specified will effect an automatic downward pH adjustment of the moist impregnated pulp to at least 8 substantially at the time when maximum brightness of the moist impregnated pulp has been reached. It has further been found that this condition of lowered pH stabilized the pulp against brightness reversion. It is preferred to employ a solution which, when applied to the moist pulp, will provide a pH of the impregnated pulp below '7 at the point of maximum brightness. However, with the semichemical pulps, the pulp pH at the point of maximum brightness may be higher without substantial deleterious effect. In this case the pH of the pulp at the point of maximum brightness might be as high as from 9 to 10. However, the lower the pH of the pulp at the point of maximum brightness, even in the case of semichemical pulps, the better the color stability.

It is to be particularly noted that during the brightening of the pulp the bleaching action takes place automatically and is self-controlling against any substantial brightness reversion so that no chemical additions or further processing is required to arrest the reversion reaction at the point where maximum brightness is achieved, the slow internal bleaching action afforded by the practice of the present invention thereby being permitted to take place without supervision or control while the impregnated pulp is in storage or is in transit.

At any time after maximum brightness is reached, the impregnated pulp can be used directly as a furnish constituent without. further chemical additions or treatment and since no brightness reversion takes place for several weeks, generally adequate time is provided for storing or shipping the impregnated pulp. It will therefore be seen that the impregnated pulp can be taken directly from storage or shipment after the lapse of any reasonable time and added directly to the furnish beater. Within the range of time provided by the present invention, this period of time adequately covers present commercial paper making practices in the use of mechanical or semichemical pulp lap as a paper furnish constituent. Further, the present invention generally provides a total brightness increase at least as great as that obtainable by the most eiiicient known processes in bleaching wood pulp containing substantial amounts of ligneous and non-cellulosic materials.

The following examples illustrate but do not limit the present invention.

Example 1 A groundwood pulp composed of spruce and balsam was thickened on a vacuum type filter to a point where a lap having a consistency of 37% solids was produced. This moist lap was then impregnated with an alkaline peroxide solution of such a concentration that 2% by weight of a 27.5% hydrogen peroxide solution, 1% by weight of silicate of soda (42 B.), and 2% by weight of commercial trisodium phosphate were added, based on a dry pulp weight, the solution having a pH of 11.0 and a total alkalinity of 0.59% sodium oxide equivalent. The impregnated pulp was allowed to stand at room temperature for varying periods of time and brightness measurements of standard handsheets prepared from the impregnated pulp were made using a Hunter Photoelectric Colorimeter. The pH of the pulp immediately following impregnation was 10.0; The pH of the pulp was 6.0 at the point of maximum brightness. The following table will serve to shOW the pulp brightness values at varying times.

Example 2 Another portion of the same pulp as described in Example 1 was treated in the same manner but the impregnating solution was such that 2.0% by weight of a 27.5% hydrogen peroxide solution, 2% by weight of silicate of soda (42 B.), and 1% by weight of commercial sodium pyrophosphate were added on a'dry pulp basis, the solution havin a pH of 10.7 and a total alkalinity of 0.67 sodium oxide equivalent. The impregnated pulp was allowed to stand at room temperature for varying periods of time and brightness measurements of standard handsheets prepared from the impregnated pulp were made on the Hunter Photoelectric Colorimeter. The pH of the pulp immediately following impregnation was 10.0. The pH of the pulp was 6.5 at the point of maximum brightness. The results also appear in the following table.

Example 3 7 was 6.0 at the point of maximum brightness. Brightness results of handsheets prepared from the impregnated lap at various periods of time in the standard manner are also set forth in the following table. Example 4 A groundwood pulp prepared from poplar wood was thickened to a point where the lap produced had a consistency of 39% solids. This moist lap was then impregnated with an alkaline peroxide solution of such a concentration that 2% by Example a neutral sulflte pretreated birch groundwood pulpi'was thickened to a point where a lap having a consistency of 27% solids was produced.

. This moist lap was then impregnated with an ,alkaline peroxide solution of such a concentration that 2% by weight of a 27.5% hydrogen per- {oxide solution, 2% by weight oi. silicate of soda (42 B.)', and 2% by weight of a commercial trisodium phosphate were added, based on .a dry -p'ulp weight, the solution having a pH of 10.8 and a total alkalinity of 0.70% sodium oxide 5. equivalent. The pH of the pulp immediately following impregnation was 10.5. The pH of the pulp was 7.0 at the point of maximum brightness. Brightness results of standard handsheets prepared from the impregnated lap at various times are set forth in the following table.

Example 6 phosphate, sodium pyrophosphate and sodium case a high increase in brightness was obtained without substantial brightness reversion.

Bactericides and fungicides may be added if desired to the wood pulp or to the bleaching solution in order to help prevent deterioration of the lap in storage or in transit.

While the present invention has been described as a single bleaching treatment of the wood pulp containing substantial amounts of ligneous and non-cellulosic material, the invention can be practiced as the last stage in a multi-stage bleaching process and particularly can be carried out following the pretreatment procedure setforth in the copending patent application of Robert L. McEwen, Serial No. 572,255, filed J anuary 10, 1945 for Treatment of Groundwood Pulp, now Patent No. 2,465,327. a

It will be apparent that changes and modifications can be made which will nevertheless fall within the scope of this invention. Accordingly, it is intended that the invention is not restricted to the various details, conditions, amounts and procedures given as typical and illustrative of preferred procedure except as necessitated by the prior art and appended claims.

We claim:

1. In the method of treating moist felted wood the pulp, said alkaline peroxide solution having -a pH of from 9.5 to 11 and a total alkalinity of from 0.35 to 1.1% by weight sodium oxide equivalent based on bone dry pulp and comprising a. peroxide and an alkaline salt including as an essential ingredient a sodium salt selected from the group consisting of silicate of soda, trisodium tetraphosphate, accumulating said impregnated An aspen semichemical pulp of 11.0% 'lignin content prepared by neutral sulfite treatment of chips followed by mechanical defibering was dewatered to a point where a lap having a consistency of 30% solids was produced. This moist lap was then impregnated with an alkaline peroxide solution so that 2% by weight of a 27.5% hydrogen peroxide solution, 2.5% by weight of silicate of soda (42 B.) and 0.5% by weight of sodium hydroxide were added, based on adry pulp weight, the solution having a pH of 11.1 and a total alkalinity of 0.65% sodium oxide equiv alent.' The pH of the pulp immediately following impregnation was 10.5. The pH of the pulp was 9.5 at the point of maximum brightness. Brightness results of standard handsheets prepared from the impregnated lap at various times are set forth in the following table.

felted pulp into a mass of substantial volume and having a relatively small exposedarea to inhibit drying out of said.mass, allowing said mass to remain in a moist felted condition for from 5 hours to several weeks during the initial period of which time the maximum brightness of the pulp' is achieved and the pH thereof is automatically lowered to at least 8 at the point of maximum brightness, and omitting any further chemical additions to the bleached pulp to inhibit color reversion thereof after obtaining said maximum brightness, the bleached pulp retaining substantially its' said maximum brightness for a, period of at least several weeks.

2. In the method of treating moist felted wood pulp containing substantial amounts of ligneous and non-cellulosis materials having a consistency of from 35 to 50% solids, the steps which comprise impregnating said moistfelted pulp with an alkaline peroxide solution as the sole chemical addition to efiect self controlled bleaching of the pulp, said alkaline peroxide solution being applied at a temperature in the range of 15 to 30 C. and having a pH of from 10.5 to 10.8 and a. total alkalinity of from 0.35 to 1.1% by weight sodium oxide equivalent based on bone dry pulp Table Ex.l Ex. 2 Ex. 3 Ex. 4 Ex. 5 Ex. 6

Untreated Pulp-...-- 50. 2 59. 2 59. 1 60. 8 45. 6 52.8 Standlng- 1 day 67. 6 70.4 57. 5 2 days 67. 0 71. 3 52. 3 3 day 71. 0 4 days 69. 5 56. 2 5 days 61.0 7 days 70. l 71. 9 71. 2 8 day 57. 7 14 days 69. 7 71. 2 68.8 70. 1 64.2 36 days 62.2

and comprising hydrogen peroxide and an alkaline salt including as an essential ingredient a sodium salt selected from the group consisting of silicate of soda, trisodium phosphate and sodium tetraphosphate, accumulating said impregnated 1a felted pulp into a mass of substantial volume and 9 having a relatively small exposed area to inhibit drying out of said mass, allowing said moist mass to remain in said accumulated condition for from 5 hours to several weeks during the initial period of which time the maximum brightness of the pulp is achieved and the pH thereof is automatically lowered from the initial pH to within the range of from about 6 to not more than 8 at the point of maximum brightness, and omitting any further chemical additions to the bleached pulp to inhibit color reversion thereof after obtaining aid maximum brightness, the bleached pulp retaining substantially its said maximum brightness for a period of at least several weeks.

ROBERT L. McEWEN.

FRED R. SHELDON.

REFERENCES CITED The following references are of record in the file or this patent:

UNITED s'm'rrzs PATENTS Richter Dec. 22, 1936 Number Name Date 2,071,305 Hirschkind Feb. 16, 1937 2,081,267 Campbell May 25, 1937 2,107,297 Kaufimann Feb. 8, 1938 2,125,634 Heritage Aug. 2, 1938 2,150,926 'Kaufimann Mar. 21, 1939 2,413,583 Shearer Dec. 31, 1946 FOREIGN PATENTS Number Country Date 351,217 Great Britain June 25, 1931 OTHER REFERENCES Paper Trade Journal, Nov. 8, 1945, pages 33 to 38.

Kaufiman, Hydrogen Peroxide in the Pulp and Paper Industry. Paper Trade Journal, Nov. 8, 1945, pages 33-38 (Tappi Section, pages 185- 190).

McEwen, "High Density Bleaching of Mechanical Pulp With Hydrogen Peroxide." Paper Trade Journal, April 25, 1946, pages 49-53 (Tappi Section, pages 179-183) Witham, Modern Pulp and Paper Making," Second Edition, 1942, page 261, Reinhold Pub. Co., New York. 

1. IN THE METHOD OF TRATING MOIST FELTED WOOD PULP CONTAINING SUBSTANTIAL AMOUNTS OF LIGNEOUS AND NON-CELLULOSIS MATERIALS HAVING A CONSISTENCY OF NOT LESS THAN 25% SOLIDS, THE STEPS WHICH COMPRISE IMPREGNATING SAID MOIST FELTED PULP WITH AN ALKALINE PEROXIDE SOLUTION AS THE SOLE CHEMICAL ADDITION TO EFFECT SELF CONTROLLED BLEACHING OF THE PULP, SAID ALKALINE PEROXIDE SOLUTION HAVING A PH OF FROM 9.5 TO 11 AND A TOTAL ALKALINITY OF FROM 0.35 TO 1.1% BY WEIGHT SODIUM OXIDE EQUIVALENT BASED ON BONE DRY PULP AND COMPRISING A PEROXIDE AND AN ALKALINE SALT INCLUDING AS AN ESSENTIAL INGREDIENT A SODIUM SALT SELECTED FROM THE GROUP CONSISTING OF SILICATE OF SODA, TRISODIUM PHOSPHATE, SODIUM PYROPHOSPHATE AND SODIUM TETRAPHOSPHATE, ACCUMULATING SAID IMPREGNATED FELTED PULP INTO A MASS OF SUBSTNATIAL VOLUME AND HAVING A RELATIVELY SMALL EXPOSED AREA TO INHIBIT DRYING OUT OF SAID MASS, ALLOWING SAID MASS TO REMAIN IN A MOIST FELTED CONDITION FOR FROM 5 HOURS TO SEVERAL WEEKS DURING THE INITIAL PERIOD OF WHICH TIME THE MAXIMUM BRIGHTNESS OF THE PULP IS ACHIEVED AND THE PH THEREOF IS AUTOMATICALLY LOWERED TO AT LEAST 8 AT THE POINT OF MAXIMUM BRIGHTNESS, AND OMITTING ANY FURTHER CHEMICAL ADDITIONS TO THE BLEACHED PULP TO INHIBIT COLOR REVERSION THEREOF AFTER OBTAINING SAID MAXIMUM BRIGHTNESS, THE BLEACHED PULP RETAINING MAXIMUM BRIGHTNESS, THE BLEACHED PULP RETAINING SUBSTANTIALLY ITS SAID MAXIMUM BRIGHTNESS FOR A PERIOD OF AT LEAST SEVERAL WEEKS. 